M
Power Source Selector for
Dual-Battery Systems
12
______________________________________________________________________________________
Detailed Description
The MAX1773/MAX1773A provide the functions neces-
sary to allow an external controller to manage the
power connections needed for two battery packs, an
AC adapter input, a battery charger, and the system
load. The MAX1773/MAX1773A use seven PMOS FETs
to provide all the switching necessary in systems using
a step-down charger powered by the AC adapter
(Figures 9 and 10). The MAX1773/MAX1773A automati-
cally adapt to many transient conditions
—
such as AC
plug-in, battery hot swapping, and battery switchover
—
to provide constant power to the system without requir-
ing real-time support from an external controller. The
MAX1773/MAX1773A draw their power from the highest
voltage supply present (Figure 11).
Battery Detection
The MAX1773/MAX1773A monitor the battery
’
s thermis-
tor voltage to determine the presence of the battery.
The devices compare the battery
’
s thermistor voltage
(V
THM_
) to the thermistor trip point (V
TCOMP
). If V
THM_
< V
TCOMP
, then the MAX1773/MAX1773A assume that
the battery is present. However, if V
THM_
> V
TCOMP
,
the MAX1773/MAX1773A assume that the battery is
absent and do not charge or discharge the battery.
Modes of Operation
The MAX1773/MAX1773A provide three modes of oper-
ation. Start-up States mode provides functionality when
the MAX1773/MAX1773A are initially powered by a bat-
tery when no AC adapter is present. AC adapter States
mode provides functionality when an AC Adapter is pre-
sent. Standard Battery States mode provides functionality
when one or both batteries are present, the AC adapter is
not present, and EXTLD is above 2.2V. The Standard
Battery States mode requires an external supply with an
output voltage between 2.2V and 4.5V for ACDET, as
shown in Figure 10. The external power supply must be
powered from EXTLD.
AC Adapter States
The MAX1773/MAX1773A check for the presence of an
AC adapter by sensing the voltage at ACDET. When
V
ACDET
exceeds the batteries
’
voltage and 4.75V, then
the MAX1773/MAX1773A use the AC adapter to power
the load. In addition, if the selected battery is present,
the MAX1773/MAX1773A connect the selected battery
’
s
charge path. See Table 1 for a detailed listing of the
MAX1773/MAX1773A states for operation with an AC
adapter detected.
Standard Battery States
When the AC adapter power supply is not present, the
MAX1773/MAX1773A use the batteries to supply the
load. BATSEL allows an external controller to select a
battery. Table 2 shows the simplified standard battery
states that normally control operation. However, the
Battery Switchover Latch, the Low-Battery Latch, and
the Discharged Battery Latch are able to suspend the
state table and provide additional functionality.
BATSEL
0
0
1
1
BATTERY A
Present
Absent
X
X
BATTERY B
X
X
Present
Absent
BATSTAT
0
1
1
0
CONNECTION STATE
AC adapter is connected to load. Battery A
’
s charge path connected.
AC adapter is connected to load.
AC adapter is connected to load. Battery B
’
s charge path connected.
AC adapter is connected to load.
Table 1. AC Adapter States
X = Don
’
t care, Present: V
THM_
< V
TCOMP,
Absent: V
THM_
> V
TCOMP,
ACPRES
= 0
BATSEL
0
X
X
X
X
1
BATTERY A
Present
Present
Present
X
Absent
X
VBATA
>5
×
V
MINV
>5
×
V
MINV
>5
×
V
MINV
<5
×
V
MINV
X
X
BATTERY B
X
Absent
X
Present
Present
Present
VBATB
X
X
< 5
×
V
MINV
> 5
×
V
MINV
> 5
×
V
MINV
> 5
×
V
MINV
BATSTAT
0
0
0
1
1
1
CONNECTION STATE
Battery A is connected to the load.
Battery A is connected to the load.
Battery A is connected to the load.
Battery B is connected to the load.
Battery B is connected to the load.
Battery B is connected to the load.
Table 2. Simplified Standard Battery States (without latches)
X = Don
’
t care, Present: V
THM_
< V
TCOMP
, Absent: V
THM_
> V
TCOMP